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SUPERPHOSPHATES                                  131

the procedure is as indicated on p. 123 for the determination of the total
phosphoric acid.    Mg2P207 X 0-64 = P205 per i gram of substance.

aqueous solution prepared as in A are added 20 c.c. of ammonium citrate,
50 c.c. of water, 50 c.c. of ammonia (D 0-92) and 50 c.c. of magnesia mix-
ture. This is shaken and treated exactly as in A.

3.  Nitrogen.—This is determined particularly in bone superphosphate
or for the detection of adulteration (see 5) by Ulsch's modification of the
Kjeldahl method (see General Methods, 3, C).

4.  Degree of Fineness.—25 or 50 grams of the superphosphate simply
mixed with a spatula (not powdered) are sieved for 5 minutes through a
sieve of 1-2 mm. mesh, the percentage passing through being determined.

5.  Adulterations.—These should be tested for with so-called bone
superphosphates, which have the greatest value and are therefore the most
often adulterated.    As adulterants, use is made more particularly of mineral
superphosphates, bone ash, bone black ;  precipitated phosphates ;  pyro-
phosphates and superphosphates obtained from them ; gypsum, calcareous
substances (chalk, powdered oyster shells) ; sand, road dirt ; various organic
nitrogenous substances (dried blood, leather or wool waste, residues from
the purification of illuminating gas).

The detection of adulteration in bone^superphosphate is not always easy
or certain. Preliminary tests to distinguish bone from mineral superphos-
phate and to detect certain other adulterants are as follows :

(a)  The sample is made into a paste with water and filtered :   bone
superphosphate gives a clear, coloured, filtrate, whereas mineral superphos-
phate gives a turbid, colourless one.

(b)  The sample is heated in a porcelain dish over an ordinary flame until
charred and then in a platinum dish over a blowpipe flame vigorously and
for a long time.    Bone superphosphates give no white fumes, but only an
odour of sulphur dioxide, and a white or faintly yellow, incandescent mass,
which is white or barely reddish when cold; mineral superphosphates give white
fumes of S03 and a mass which is yellow when hot and brick-red in the cold.
Further, the residue from bone superphosphate is completely, or almost
completely, soluble in hot 10% hydrochloric acid, whilst mineral superphos-
phate leaves a more or less abundant residue insoluble in the dilute acid.

(c)  The aqueous solution of the sample is tested for chloride, a marked
proportion of the latter indicating the presence of precipitated phosphate.

(d)  Abundant effervescence when the sample is treated with hydrochloric
acid indicates addition of calcareous substances.

A complete examination requires, however, systematic investigations
and determinations including : microscopic observations or a petrographical
study, determinations of the total and citrate-soluble phosphoric acid, sul-
phuric anhydride, silica, insoluble residue, nitrogen in the latter, fluorine,
chlorine, lime, alumina and ferric oxide and manganese.1

1 See E. Lasne : " Detection of Adulteration of Bone Superphosphate " (Staz. sper.
agrar. ital., 1898, p. 270) ; F. Martinotti: "A Method for Distinguishing Bone Phog
phates from Mineral Phosphates " (ibid., 1897, p. 663); G. Masoni: " Contribution to^me
Detection of Adulterants in Bone Superphosphate " (ibid., 1910, p. 297).ranhfonn into phosphoric at id the pyrti ami meta-pho.Hplwric